1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., photoresist or resist). Instead of a mask, the patterning device may comprise an array of individually controllable elements which serve to generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
When producing a pattern on a substrate, it is necessary to ensure that the patterned beam of radiation is correctly aligned with the surface of the substrate onto which the pattern is to be produced. This may be necessary to ensure that separately generated portions of the pattern are correctly aligned relative to one another, such that the overall pattern produced is correct and does not have discontinuities at the boundaries between the portions. Alternatively, it may be necessary to ensure that a pattern produced on the substrate is correctly aligned with a pattern previously produced on the substrate, for example, with another layer in a device with a plurality of layers. Conventionally, this has been achieved by mounting the substrate on a substrate table, accurately measuring the position of the substrate relative to the substrate table and subsequently carefully controlling the position of the substrate table until the substrate is in the desired position.